1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and more particularly to an image transferring device configured to transfer color images to both sides of a recording medium substantially at the same time without switching back the recording medium, and an image forming apparatus including the same.
2. Description of the Background Art
Generally, two different systems, i.e., a switchback system and a one-pass system are available for transferring toner images to both sides of a sheet or similar recording medium. The switchback system conveys a sheet via image transferring means to thereby transfer a toner image from a photoconductive drum or similar image carrier to one side of the sheet, switches back the sheet, and then transfer another toner image to the other side of the sheet. On the other hand, the one-pass system transfers toner images to both sides of a sheet with image transferring means substantially at the same time without switching back the sheet. The one-pass system is advantageous over the switchback system in that it is free from an increase in cost and an increase in image forming time ascribable to a sophisticated switchback mechanism. An image forming apparatus implementing the one-pass system is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 1-209470.
We are currently developing an image forming apparatus using a first and a second intermediate image transfer body as an improvement over the conventional one-pass system. In the improved apparatus, the first and second intermediate image transfer bodies are held in contact with each other and caused to endlessly move while forming a nip therebetween. When a sheet nipped by the above nip is being conveyed toward a side downstream of the nip in the direction of belt movement, a first toner image transferred from an image carrier to the second intermediate image transfer body via the first intermediate image transfer body beforehand is transferred to the first side of the sheet. At the same time, a second toner image transferred from the image carrier to the first intermediate image transfer body beforehand is directly transferred to the second side of the sheet. Because the sheet does not directly contact the image carrier, paper dust, when a paper sheet is used, does not deposit on the image carrier or bring about troubles including the degradation of image quality.
However, the improved apparatus described above is apt to cause a sheet to crease at the time when toner images are transferred to both sides of the sheet. It was experimentally found that creases were conspicuous particularly when the first and second intermediate image transfer bodies each were implemented as a belt passed over a plurality of rollers and caused to endlessly move. We found the following as a result of extended researches and experiments. Generally, the first and second intermediate image transfer bodies are configured to exhibit elasticity, so that they can closely contact a sheet for thereby enhancing transferability. However, when the two intermediate image transfer bodies with high elasticity are pressed against each other to form a nip, their surfaces elastically deform in a complicated manner at the nip and bite into each other in the form of wedges. When a sheet is conveyed via such a nip, it creases in accordance with the configuration of the creases.
Japanese Patent Laid-Open Publication No. 9-258518 discloses an image forming apparatus including a single photoconductive drum and a plurality of developing units arranged around the drum and each storing toner of a particular color. First, exposure and development are repeated color by color with the drum to thereby form a composite color toner image for the first side of a sheet on the drum. The color toner image is then electrostatically transferred from the drum to a first intermediate image transfer body and then electrostatically transferred to a second intermediate image transfer body. Subsequently, a composite color image for the second side of the sheet is formed on the drum by the same procedure and then electrostatically transferred to the first intermediate image transfer body. Thereafter, the toner images are electrostatically transferred from the second intermediate image transfer body or acceptor and first intermediate image transfer body to both sides of a sheet. In this manner, the apparatus electrostatically effects all of the consecutive image transfer.
Generally, electrostatic image transfer is desirable when effected at a position where a sheet and an image carrier closely contact each other. However, at a position where the sheet and image carrier do not closely contact each other within an image transfer zone, electrostatic image transfer brings about toner scattering or blurring due to discharge, which occurs in the event of contact and separation of the sheet, and the influence of an electric field. The resulting images lack sharpness.
Further, Japanese Patent Laid-Open Publication No. 2000-250272 teaches a tandem image forming apparatus including four photoconductive drums arranged side by side, a first intermediate image transfer belt or body contacting the drums, and a second intermediate image transfer belt or body selectively movable into or out of contact with the first intermediate image transfer body. First, toner images are formed on the drums in accordance with image data read from the first side of a document while being sequentially transferred to the first intermediate image transfer belt one above the other, completing a color toner image. The color toner image is then transferred to the second intermediate image transfer belt by heating means associated with the first intermediate image transfer belt. Subsequently, another color toner image derived from image data read from the second side of the document is completed on the first intermediate image transfer belt. When a sheet is conveyed to a position between the first and second intermediate image transfer belts, the color toner images are transferred from the first and second intermediate image transfer belts to both sides of the sheet and fixed on the sheet at the same time by the heating means associated with the first intermediate image transfer belt.
The apparatus taught in the above document applies electrostatic image transfer to primary image transfer from the drums to the first intermediate image transfer belt and applies thermal image transfer to secondary image transfer from the first intermediate image transfer belt to the second intermediate image transfer belt and image transfer from the two belts to both sides of a sheet. With this scheme, it is possible to reduce toner scattering and blurring ascribable to electrostatic image transfer. However, because the toner images are fixed on both sides of a sheet at the same time as they are transferred from the two belts, the two belts are heated to high temperature. It is therefore necessary to use cooling means for cooling off the two belts after image transfer to temperature below the softening point of toner. The apparatus is not practical when applied to a high speed machine because it cannot save energy and because the cooling means increases the overall size of the apparatus.